This invention relates generally to the measurement of light scattered from a surface or volume and the use of the angular intensity spectrum of this scattered light for characterization of the microstructure of the surface or volume. More specifically, this invention measures the intensity of light using two-dimensional detectors or detector arrays. As used herein, the term microstructure refers to the morphology of the sample and the manner in which the morphology is distributed at different spatial frequencies or spatial wavelengths (i.e. structure of different lateral extent). Microstructure refers to surface as well as volume morphology of the sample.
Scattered light can be analyzed via well known models to provide a statistical analysis of the microstructure of a surface or volume sample from which the scattered light originates. This provides a simple, noncontact, and nonperturbing monitoring technique which is useful in many areas of technology to determine surface and subsurface morphology. In addition, the type and density of material defects which have simple geometric shape can be characterized using this technique. This technique is useful in such areas as microelectronics material fabrication, optoelectronics material fabrication, optical component examination, and computer disk manufacturing. Light scatter measurements are also useful for quality monitoring of fluids. For example, blood samples can be conveniently examined using light scatter techniques to reveal cell characteristics. Other fluids with particulates or small specimens held in suspension, such as oils, biological specimens, wine, gas containing particulates, and the like can be conveniently examined using light scatter measurements. Moreover, these measurements can be made in-situ to control processes used in the manufacture of the various materials described above.
A commonly used scatterometer system employs a laser light beam incident on a point of a sample and a single light detector that is mechanically rotated or scanned in an arc contained in the plane of incidence, defined by the incident and specularly reflected light beams that are centered about the point on the sample. The intensity of light scattered by the sample is measured sequentially at selected scatter angles, relative to the normal of the sample. The pattern of the scattered light intensity is then analyzed to obtain characteristics of the microstructure of the sample. Similarly, an array of detectors can be located in an arc or line segment, contained in the plane of incidence, to measure scattered light. This configuration yields the same information as the single-detector configuration, but it does so in a more efficient manner by performing a number of measurements simultaneously, rather than sequentially. Other detector configurations are known in which one detector array measures the scattered light intensity pattern in an arc or line segment located in the plane of incidence, and another detector array measures the scattered light intensity pattern in an arc or line segment located in the plane perpendicular to the plane of incidence containing the scatter point of the sample.
A shortcoming of prior art scatterometer systems is the difficulty associated with characterization of the light scattered from the sample which lies out of the plane of incidence, defined by the incident and specularly reflected laser beams. It is often desirable to characterize all of the light scattered from a sample, thereby obtaining a two-dimensional map of the scattered light intensity pattern. This is especially the case for samples which have nonisotropic microstructure and therefore scatter light nonisotropically. Examples of samples having nonisotropic microstructure include, but are not limited to, machined surfaces, single and polycrystalline materials, crystalline materials with well known defects of specific geometric shapes, microelectronic integrated circuits, electro-optic and ferroelectric materials, fluids, and fluids with materials dissolved or held in suspension. To fully characterize scattered light from a sample using prior art systems to thereby obtain a two-dimensional map of the scattered light intensity, the sample must be incrementally rotated about an axis perpendicular to the sample and passing through the point which is illuminated. After each incremental rotation, the scattered light intensity pattern is measured along an arc length. Thus, many measurements must be made sequentially to fully characterize the scattered light, resulting in a tedious and time consuming process. Alternatively, the detector may be configured such that it is capable of rotating in an arc that is out of the plane of incidence. However, this configuration adds greatly to the cost and complexity of the scatterometer system.
Known two-dimensional detector arrays have been used in conjunction with other optical elements such as lenses and apertures forming a camera. Other scatterometer systems employ only a camera as a detector element. In this arrangement, light which is scattered near the specularly reflected beam, known as the near-angle scattered light, is characterized. Similarly, the camera might be located so as to be significantly removed from the incident and specularly reflected laser beams. In this configuration, a portion of the large-angle scattered light from the sample is characterized. These configurations are significantly limited in the amount of scattered light that they characterize compared to the total amount of light scattered from the sample due to their small angular field of view.
It is therefore a principal object of the present invention to provide an improved detector system for optical scatterometers in which light specularly reflected and scattered from a sample is used to obtain a two-dimensional intensity distribution and thereby characterize the morphology of the sample in two dimensions of spatial frequency.
This and other objects are accomplished in accordance with the illustrated preferred embodiments of the present invention by employing a screen positioned to receive and display a pattern representative of light specularly reflected and scattered from an illuminated sample material and a camera positioned to record the pattern displayed on the screen. The screen may present a curved surface to increase its light gathering capabilities and may be constructed of an electron trapping material, a photochromic material or a pyrochromic material. The screen may contain one or more apertures for passing one or more incident light beams generated by a light source and/or light specularly reflected and scattered from the sample material. The screen may be positioned between the sample material and the light source, or the sample material may be positioned between the screen and the light source.